1. Field of the Invention
The present invention relates to an improvement in a delayed coking process.
2. Description of the Prior Art
Delayed coking is a well-known process in which a hydrocarbonaceous oil is heated to a coking temperature and then passed into a coking drum to produce a vapor phase product, including normally liquid hydrocarbons and coke. The drum is decoked by hydraulic means or by mechanical means. In some configurations of the delayed coking process, the fresh hydrocarbonaceous coker feed is introduced into the coker product fractionator, usually for heat exchange purposes, where it combines with the heavy coker products that are recycled to the coker heater. See Hydrocarbon Processing, September 1980, page 153.
The delayed coking process is generally conducted at a temperature ranging from about 800.degree. to about 950.degree. F. Typically delayed coking is conducted at a temperature above about 900.degree. F.
U.S. Pat. No. 4,057,487 discloses a fluid coking process in which the heavy oil stream separated from the coker products in a scrubbing zone is passed to a vacuum distillation zone and a vacuum distillation zone bottoms fraction is recycled to the coking zone. Although such a scheme decreases the amount of lighter materials of the scrubber bottoms that needs to be recycled to the coking zone, there still remains a significant amount of relatively low Conradson carbon content constituents in the recycle stream.
U.S. Pat. No. 2,777,802 discloses, in FIG. 2, a fluid coking process in which the total coker overhead product is subjected to extractive distillation to remove metal contaminants. A bottoms product from the extractive distillation zone may be recycled to the coking zone.
U.S. Pat. No. 2,943,995 discloses a two-stage coking process in which a gas oil fraction separated from the first coking stage product is extracted with a solvent selective for aromatics, and the extracted aromatics are coked in the second coking stage.
U.S. Pat. No. 3,247,095 discloses coking a reduced crude and recycling fractionator bottoms to the coker. As shown in FIGS. I and II, a coker gas oil is withdrawn and solvent extracted. The extract is stripped and the stripped extract is recovered.
U.S. Pat. No. 3,172,840 discloses a delayed coking process in which the effluent in FIG. I is fractionated. A gas oil side-stream from the tower is thermally cracked and recycled to the coke drums.
It has now been found that withdrawing a portion of heavy hydrocarbons from the coker product fractionator before it can mix with the fresh oil feed that is being introduced into the fractionator, subjecting the withdrawn portion of heavy hydrocarbons to solvent separation, recovering at least a portion of the lower Conradson carbon product, and recycling the high Conradson carbon product to the coking zone will produce advantages that will become apparent in the ensuing description.